Regardless of whether it’s worthwhile designating the “Anthropocene” as a new geological epoch (I have my doubts), determining the trajectory of human impact upon the Earth system is important. It provides context for how people see their relationship with natural systems and resources, and can help shape environmental policy and its acceptance by the public. But where to put the onset of this impact – where does the Anthropocene start? The authors of this paper belong to the Institute of Social Ecology at Klagenfurt University, Austria, and take a social sciences approach to the problem. Rather than focusing on physical evidence in the environment, as a geologist might do to delimit a geological epoch, Fischer-Kowalski et al. use a model where human impact is measured as the product of population size, affluence (= energy available per person) and technology, summed over three modes of subsistence: hunter-gatherers, agrarian and industrial. This avoids problems of time lags between human activity and the signature it leaves, and allows the authors to pull apart different driving factors across different societal types.

Using this approach shows a definite shift at around AD 1500: human impact was gradually increasing prior to this, but there is a sharp upturn at ~AD 1500, energy use becomes more important for amplifying the impact of population growth, and a shift from biomass to fossil fuel driven energies enhances this further. While there are problems with this approach – determining energy throughput for long-dead societies will always involve a lot of extrapolation from modern patterns – the authors are careful in stating their assumptions, and going through possible issues with the model itself. It would have been nice to see a comparison between the model output and the empirical evidence though, to help bridge the gap between these two different takes on the same question.